i) Field of the Invention
This invention relates to a thermoplastic, hydrophobic nanocomposite of nanocrystalline cellulose (NCC) and a hydrophobic vinyl monomer, and a process for producing such a nanocomposite; the process may be considered a green process. The nanocomposite is a sustainable, thermally-stable biomaterial which is hydrophobic and compatible with a wide range of synthetic polymers. The nanocomposite material can be suspended in appropriate solvents, as well as dried and moulded with other materials using conventional polymer processing techniques to develop yet more materials with new characteristics. These nanocomposites have wide ranging applications from industrial to medical use.
ii) Description of the Prior Art
It has been recognized that nanocrystalline cellulose (NCC) can be applied as a reinforcing material in nanocomposites. NCC, a chiral nematic structure of high strength and elastic modulus, is renewable, has a relatively large reactive surface, and is biodegradable. The major obstacles to NCC application in composite manufacture are (1) the dispersion of hydrophilic NCC in mostly hydrophobic polymer matrices and (2) the poor interfacial adhesion between NCC and the polymer. Different approaches have been followed to increase NCC's dispersion and interaction with polymer matrices. NCC has been either coated with surfactant or chemically surface modified.1,2 Use of surfactants is a simple enough method, but a large amount of surfactant is normally required which would negatively impact the strength of the resulting composite. Surface modification, on the other hand, generally involves reacting with the hydroxyl groups on the NCC surface. Silanes have been employed to graft hydrophobic groups onto the NCC surface. Moreover, some polymers with reactive groups for hydroxyls have been used as well, such as PEG,3 PCL4 and PP.5 These modifications can make NCC more hydrophobic and give NCC reasonable stability in organic solvents. However, these reactions are generally performed in organic solvents and involve several steps.
Surface graft co-polymerization of vinyl monomers onto polysaccharide surfaces, e.g. cellulose, starch, and chitosan, has been investigated extensively.6-8 Methyl methacrylate, methylacrylate, acrylic acid or vinyl acetate can be used as the monomer of choice. Graft polymerization has been reported using different initiation systems, including photo initiators, UV light, γ-ray radiation, free radical initiators, etc. In these techniques, free radical initiators, such as AIBN, iron(II)-hydrogen peroxide, potassium persulfate, and transition-metal ions, have widely been used. Among the free radical initiators, ceric (IV) ion has shown great efficiency in grafting vinyl monomers onto a number of polysaccharides. The proposed mechanism for the initiation reaction has been ascribed to the formation of a complex with the hydroxyl groups on the polymer backbone, which can dissociate via one electron transfer to give free radicals.